The present invention relates to clamp apparatus of the kind comprising a housing presenting a base, a disc journalled within the housing for rotation about an axis parallel to the base in cylindrical radial bearings solid with the housing, the disc being provided with a clamp arm projecting through a slit in a wall of the housing above the base, the slit being juxtaposed with the disc, and the disc being provided with a toothed sector generally diametrically opposite to the clamp arm and cooperating with a tangential screw disposed in the housing substantially perpendicular to the base, a fixing member on the base for fixing the housing to its support on the machine tool, and a scraper blade disposed on the opposite face of the housing juxtaposed with the circumference of the housing to close the housing against ingress of foreign matter.